Pulse time modulation multiplex receiver



Aug. 17, 1948. P. K. CHATTERJEA ETAL' 2,447,233

PULSE TIME MODULATION MULTIPLEX RECEIVER [nu ntors A ttorn ug 17, 1948.-P. K. CHATERJEA lai-Al. 2,447,233

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l PULSE TIME MODULATION MLTIPLEX RECEIVER Filed March 15, 1944 v 'r,sheets-sheet 7 2nd. BUILD AcK C] BEH- Asc Ell/e7- FILTER W A ttorny-Patented Aug. 17, 1948 Y A .2,447,233 PULSETIME M O DEKIIKJA'LLION'MULTIPLEX 'RECEIVER Pra'fulia -Kuinar Chatterjea and Leslie WilfredEngland. .assieme t0 Standard Telephones and Cables Limited, Lon- I -Y-don,England,-a Brit-ish company y Y Y Application March-515, 19454,Sria-*l No. 552.6;5'66 v,

Greatritain 'April '1, 194'3 "l The present invention relates to'multiplex `signal high frequency `'transmission systems "ojf "the kindutilising electrical pulses.

It is now Well known that signal Waves bear*- inff Vintelligence can loe'transmitted by means ci.. a train of 'electrical pulses 1occurring fatequal time intervals and whose dura-tions are varied in vaccordance withthe instantaneous amplitude of the signal. Such pulses are 'hereinreferred to as duration modulated pulses. In 1a modification of thissystem of signal Wave transmission, shcrt electricalpul'sefs'cffcoiistant'duraticn are transmitted to mark the lleading andtrailing edges of duration 'modulation Apulses,'and Wlre'n either theleading or trailing Yedge occurs at'equal intervals of time, onlythepulse Amar-King 'the edges of the duration trnodulated pulses Whichoccur at varying -intervals of timefneedibe trans*- mitted. This type oftransmission `is 'sometimes referred to as ia single pulse or time'modul-ated pulse transmission 'cr 'pulsed phase'fmdulation, the pulseso1" constant -short duration `occurring at varying intervals o'f time inaccordance with the instantaneous amplitude of thesignalrwave to betransmitted.

When both the 4ixed and -movirig'f'malrking pulses of a durationmodulated 'system 4are'traitsinitted, the system is referred lto 'as yafdoilble pulse system. In the single pulse system, :the Xed pulse may berei'n'sert-e'd `at 'the receiver and the solid pulses dr'duatnmdulatedpulses obtained from `the 'iixed movable ,.'pa'irs *of Lcn'don,

it claims. (otite-i5) l timed orfphasedto-occur pulsesfor examplelcymearis'cf 'knownfbuildback circuits which serve `to rproduce theoriginal rectangular duration yn'ioclulated pulses. It 'in the singlepulse sys-tem the pulse duration lis -only a small "fraction of the timeinterval between the occurrence oi 'successive pulses. fafnd 'if the.variation of this time interval 'taken up lfor 'fa manie inuin time crphase modulation of the pulses is also small, then other trains ofsimilar zp'ulses may be transmitted having 'the fsanie'pulse repeltiticn frequency, but vvhselpulses occur'in the intervals between thepulses 'of f'ano'tlier train?. Thus each pulse train `may lhear time orAphase 'modulation of its respective 'sign'f'a'l 'wave and 'a multiplexchannel transnisscn v'syste'ni is prbvided. It 'is the object ofthisin'ventio'n to p 'ro' vide 'practical arrangements 'whereby such `lasystem may be put into practice.

In systems of this type fo'r main 'operations are essential. Firstly,producing the Gramsci unn'iodulated or uniphase'd pulses, one train "foreach channel, the pulses o'f ach train idr-each channel, the nuisesio'feach being corrctiy 2 with respectto the` pulses 'voi other tra-ins ofthe system, and modulating the `.pulses of each chan-nel Awith thevrespective signal Wave. Secondly combiningall the channels into onetransmission. Thirdy, -at the re '-ceiving enf-lof the system directingthe pulsesgoi a channel to respective receiving apparatus. Fourthly,obtaining the signal Waves from the ytrain o-fpulses -in fthe respectivechannel. v twill be understood that Amore than one of thesex-opera-tions may be performed by the same equipment; that `thepulses-may be transmitted by line, Wave guide or radio,` and `maycomprise di- 1rect currentnpulses 'or trains oiwaves `of higherireq-uency than the pulse repetition frequency.

-Apparatus for' carrying out these loperations in practice Will now icedescribed, yreference being made to the accompanying drawings.

-In the drawings-z n Fig. -l is an. explanatory 'diagram relating to anVarran'gement for producing timeaphased lpulses;

Fig. 2 shows in blockischematic a circuit arrangement `of a`multi-channel pulse :signalling Fig. '3 is lan v errplainatory diag-ramused in con nection with Fig. 2; n r Y 'Figxfi shows in block schema-ticanother `circuilt Karrangement :of la lr'nulti-channel pulse signallingsystem; v

`Fig. 5 :shows "in blockschematic a lfurther circuit farraiigern'ent ofTa multi-channel Vpulse signalling Isystem;

xFig. =6 is an Fexplanatory `diagram used in ccnlu Anecticn with Fig. 5;Y

vFig -'7 vshows diagrammatically a cathode ray tube arrangement forproducing time-phased modulated -pulse's in "a'lplurality off channels;

Fig. 8 rshows lin block :schematic vform circuit arrangements fof 'areceiving equipment; Fig 9 -sliiovsexplariatcry curves used in `thedescripti'onf AFig. 8; Y

Fig. "1a 's'hvvs 'in block schematic Afunn Athe Are Caving circuit 6rrtg, 1.1 incftran'ng corrective devices in accordance vvt'h features ftl'ieinm vention; A

Figs. I3 and 14 show various curves used in -t-he description; Y

Fig. 15 shows lin `block schematic form receiving arrangementsincorporating further corrective devices;

Figs. 16 and 17 are explanatory data;

Fig. 18 shows curves used in the explanation of the operation of thecorrective devices used in Fig. l;

Fig. 19 is a circuit arrangement of the unit |05 of Fig. 15;

Fig. 20 shows a circuit arrangement of the systems shown in Fig. 15.

From one aspect, a system of the type specified comprises a series ortrain of short constant duration pulses, every nth pulse of which istime or phase modulated in accordance with a desired signal wave, therebeing n different channels. From another aspect, a system of the typespecified comprises a plurality of pulse generating devices of the samepulse repetition frequency, one for each channel, and which are properlytimephased with respect to each other so that the generated pulses ofone device or channel occur during the intervals between the generatedpulsesof the other devices or channels, the duration of the pulses andthe intervals between successive pulses of the combined channels leavingsufficient time between successive occurrences of pulses in the combinedchannel train to allow for time modulation of the pulses.

The pulses of a plurality of channels are shown in Fig. 1 of thedrawings. By way of example, four channels are illustrated. Channel Iconsists of pulses a, a1, all, etc. Channel 2 consists of pulses b, b1,D11, etc., and so on for channels c and d which are representative ofchannels 3 and 4 respectively. Each pulse is liable to occur at anyposition within the time interval indicated by Yits adjacent dottedlines, depending on the amplitude of the modulating signal.

vArrangements for producing a time-phased pulse train for one channelwill now be described.

Firstly, a train of pulses are duration modulated according to knowntechnique, and difierentiated for -example by passing through a highpass lter to produce pulses of short constant duration of opposite signsrespectively at the instants of the leading and trailing edges of theduration modulated pulses.

There are many ways of phase modulating a pulse train, and similarlythere is also a variety of methods which may be employed forcombiningindividually modulated pulse trains, and/or the producing of pulsetrains having the correct unmodulated relationship prior to their beingmodulated by their respective modulating signals. A basic circuitarrangement is shown schematically in Fig. 2 where I represents asinusoidal oscillator of the pulse repetition frequency of each channel.Its output is fed to units 2, 3, 4 and 5, which phase the sinusoidalwave the correct amount in each channel corresponding to the unmodulatedrelative pulse phases of the different channels as shown at a, b, c andd Fig. 3. Units 6, 1, 8 and 9 are further phasing circuits in which thedegree of phasing is controlled by the applied signal voltage applied atterminals I5, I6, I1 and I8, respectively, according to known practicesuch as is employed in frequency modulation systems. If desired, thesetwo stages 2-6, 3--'l, 4 8 or 5 9, may conveniently be combined. UnitsI0, Il, I2 and I3 are pulse forming circuits for example biassedamplifiers and amplitude limiters which convert the applied phasedsinusoidal waveforms into short duration pulses, which pulses from allthe channels are fed to a common amplier so as to form the requiredsingle pulse Itrain in amplifier or other unit I4. This whole operationis further illustrated by Fig. 3 where a. represents the output of I,Fig. 2. It is usual though not necessary to allow this to provide one ofthe channels so that here no phase displacement is given by 2, Fig. 2.Phase displacements given by units 3, 4 and 5, Fig. 2, provide outputsas shown at b, c and d, Fig. 3, respectively.

Neglecting for the moment the variable phase shifting produced by themodulation, signal units IB, Il, I2 and I3 produce pulses as shown at e.f, y and h, Fig. 3, which on combining at unit I4, Fig. 2, produces thepulse train y, Fig. 3, which may then be employed to modulate a carrierwave transmitter according to known practice.

Another basic arrangement for producing the phased pulse trains is shownin Fig. 4 where I9 is any pulse generator and 20, 2|, 22 and 23 are timedelay networks which may each be divided into two sections 20a, 20h,2Ia, 2lb, 22a, 22h, 23a. and 23h, respectively, if desired, parts 20A,etc.,

Vgiving the requisite fixed delay so as to phase the pulses of eachchannel appropriately and parts 20B, etc., providing a small Variabledelay, controlled by the modulating signal which would be? applied atterminals 24, 25, 23 and 21 respectively. Such variable time delaydevice may, for example, comprise an artificial line network having anelectron `discharge device arranged so that by means of its reactanceVariation, known as the Miller effect, the inductance or capacity of thenet work can be varied to modify the delay produced by the line network.The Miller effect is controlled in known manner by the signal voltages.This circuit may be of any well known form as is used in ordinary phasemodulation circuits, it being only necessary to substitute a pulset-rain for the usual sine wave input of such phasing units. A suitablecircuit for this purpose is illustrated in U. S. Patent No. 2,259,392,grantedV October 14, 1941. Then these pulse trains would be combined atamplier 23 and the resulting pulse train obtained at terminal 29 usedfor modulating a transmitter.

A third basic arrangement for producing the phased pulse trains isillustrated by Fig. 5 where 3|] is a sawtooth generator or generator oiany waveform from which it is possible to produce a variable widthpulse. Y This is then fed into pulse former units 3l, 32, 33 and 34respectively each containing an amplifier, and these ampliers are eachbiassed or otherwise adjusted so as to pass varying portions of the waveform and thus to produce progressively longer duration pulses fromchannel to channel (see Fig. 6). By applying also a modifying voltagedetermined by the channel modulation signal to terminals 35, 36, 31 and38 respectively, operating for example on the amplifier kcontrol grids,these various width pulses may be duration modulated andthcn afterdierentiation by units 39, 4B, 4I and 42 which comprise, for example,high pass lters and amplitude limiting by units 43, 44, 45 and 46respectively, to eliminate one of the differential pulses, thedifferential pulses due to the moving edges of the duration modulatedpulses are obtained and are the required phased pulse trains which maybecombined together in amplier unit 41 prior to being used to modulate atransmitter. If the initial waveform is a sawtooth the resulting pulseswill have one xed edge the differential pulses resulting from which areeliminated, and one variable edge, but in the case of, say, asinewave'both edges will vary. However, this and-3b the rectified mea-n.voltage. ItI will-bessen that pulses-` |140Bjcorresponding totransmitted pulses,l I 40; may Ybe easily segregated 'by amplitudeclipping to provide .the lsynchronizing waves. -gThe same'efectfcanofcourse be obtained by omitting Vone channeL: a. negative 'dip beingobtainedasshown dotted at b, Fig. 9, which may be usedasthesynchronisingp'ulse. f- 1 Y iis-fAn entirely different method ofreception 'makes use of-.thadoublepulse build backcircuit `principle andmay be termed Progressive selection method. HA-build backfcircuit is amultiavibratoritype :circuitwhich-,has a free `.oscillation period atleast-asgreat as vthe greatest pulse sepa- .rationand has, doublestability, or in other words, two, conditions ofrest, and-,isgarrangedto remain in either condition of rest until acted upon by a pulse,whereupon it shifts to theother condition otrest `in which condition itremains until acted uponibyapulseof the same polarity as the previouspulse, whereupon it shifts backV again to its first condition of rest.The method using such a circuit consists in passing the received pulsetrain obtained from the receiving unit after rectiilcation of the H. F.carrier signal, through a vb uildback'circuit with the result thatalternate pairs oi pulses form alonger duration pulse, the leading edgebeing formed by one pulse and the trailing edge bythe next, the position.of these edges being dependent upon the positions of the correspondingpulses. On passing this newly formed pulse through a diierentiatingcircuit a positive and a negative pulse will be obtained from therespective edges and these may readily be separated vby passing throughla limiter stage. This therefore, has the result of separating thereceived pulse train into two separate trains. A further division ofthese two trains may now be carried out until eventually pulse trainscorresponding to `individual channel pulse trains are obtained.Theoperation of this arrangement may be rreadily seen by referring toFig. l where e represents a received pulse train containing for examplefour channels, namely a, b, c and d, channel a consisting of pulse a,a1, al1, etc., channel b of pulses b, b1, b11,et etc. Y v

- On feeding this pulse train e to a build back circuit the pulse trainshown at f is obtained which on passing through a differentiatingcircuit, for example a high pass iilter, produces pulse train gconsisting of alternate positive and negative pulses from whichbypassing through correctly adjusted limiting circuits may be obtainedpulse trains hand i. O-n repeating these operations with build-back anddiierentiating circuits utilising the pulse train shown at h, pulsetrain y is obtained and thence Vpulse trains 7c, l and m from which itwill be seen that pulse train l consists of the pulses Yforming channela only i. e. a, a1, a11,etc. and pulse train m consists of channel' cpulses only namely c, c1, etc. A similar operationparried out utilisingpulse train i would result in the separation of pulses of channels bkand d.

The great advantage of this system of reception is that no synchronisingmechanism need be employed at Vthe receiver but the number of channelsrequired in this system must be 2 where 11, can have any integral valuebetween 1 and inlnity. A schematic circuit arrangement for such a systemis shown in Fig. 1l in which 12 represents the receiving unit from whichthe multi-channel pulse train is obtained. 13 represents the first buildback circuit and differentiation circuit followed by limiter circuits 14and 15 respectively adjusted to pass vOli-'ly one o ffthe differentiatedpulses from the output of 13. Each Vof these then feeds intothesecondbuild back and differentiation circuits 16 and 11 respectively,from whichthe two further pulse -trains forming the required channel4pulse trains are obtained via amplitude limiter circuits 18, 19, and 8lat terminals B2, 83, 84 and 85 respectively. Whilst an aerial pickup hasbeen shown in Fig. 11 it will be understood the pulses maybereceivedover a transmission line. p

More specific circuit detailsywill be disclose hereinafter in connectionwith Fig. 20.-vr` Y Y The demodulation oftheA individualchannel pulsetrains may be performed by the employment ofa circuit as described inthe specification of United States application Nlo. 374,660, issuedPatent No. 2,406,790, September 3,1946, or by any other known pulsedemodulation circuit such as a low pass lter. The circuit disclosed insaid application `is a differentiating Ydetector circuit having a threeelement discharge tube provided with a grid leak and condenser inputcircuit. The Whole system may be made to include all the advantagesinherent in pulse systems of transmission such as improved signal tonoise ratio and economical power consumption. With the synchronisedsystems of reception each channel is made unresponsive to any unwantedsignal such as noise interference except during the period at which thewanted pulse may occur. In the case of the progressive selection method,by operating all amplifiers in the pulse circuits from anode current cutyoi to overload or saturation value, unwanted signals are in this casealso eliminated except for the period occupied by the pulse edge as withall pulse systems of transmission.

In its basic form the progressive selection system just described isliable to produce unwanted results if the received signal is notentirely free, especially where radio links are employed, from externalinfluence such as interference or fading etc. or intentional jamming Ybyinsertingextra pulses to change the recurrence frequency.

Under such circumstances it is possible that any individual channelsignal may appear at different selecting circuits in a random fashion.

The-following are the methods and arrangements according to features ofthis invention by which such occurrences may be prevented--it will berealised that these methods will also secure the correct startingoperation or selection, in proper rotation, of the channels of thecommunication system.

In describing the following methods it will be assumed that such detailsas limiting, differentiating and other such items will be inserted wherenecessary, `as will be understood by those versed in the art. Thedescription will first Vbe oi the principles of methods which may beused. These can be enumerated as follows:

(a) Correction of functioning may be obtained by the use lof build backcircuits oi the multivibrator type wherein their freel or naturalrunning frequency periods correspond to maximum permissible timemodulation of the received pulse train in the manner shown in Fig. 13 inwhich a, b, c, etc., represent the pulses,V al, a2, b1, b2, c1, c2,etc., the respective limitsof time modulation. The second curve in thisfigure shows the pulses when the multivibrator circuit is running freeat the pulse repetition frequency of each channel multiplied by thenumber of channels, the pulses having the maximum durations. The thirdcurve shows the pulses in the multivibrator outq ..9 put circuit'whencontrolled by signal pulses, there being no external eiTect to upset theproper and correct reception. To ensure correct occurrence at equallyspaced intervals of time, the leading edges of the pulses would besynchronised to a local master oscillator, which would in turn besynchronized with the average repetition frequency of the incomingpulses. Trigger pulses from the master oscillator may be suitably phasedin a known manner and used t operate the multivibrator circuit in onesense to produce the leading edges of the pulses shown in the thirdcurve of Fig. 13, while the incoming signal pulses are used to Ioperatethe multi-vibrator in the other sense to produce the trailing edges ofthe pulses.v

(b) By converting the time interval of the interference into anamplitude transient which latter may then be used to provide therequired correction. For example, referring to Fig. 14, a train of sharpsignal pulses would produce in a circuit a mean D. C. level of height hmwhichin the absence of external eil'ects will be constant but when anyexternal interference arrives the value .of hm will vary and thisvariation may be used to provide the required correction, asby utilizingthe value hm to control the blocking of the receiver so that theinterference p-ulse will block the receiver and prevent the response ofthe build back circuit at that instant.

(c) A distinguishing feature may be given to one channel pulser trainsuch as a pilot frequency modulation fc which may occupy the samechannel as Ya Ysignal or may occupy a channel itself. Alternatively onechannel pulse train may consist of pulses of a duration different fromthe pulses in the remaining channels. Other methods not so satisfactoryconsist in giving the pulses in one channel a different amplitude or adifferently shaped edge from the pulses of other channels.

Use is then made of this distinguishing feature or channelcharacteristic at the sections of the system where pulse trains areseparated by providing an additional unit capable of indicating whetherthe special pulse train is in its correct path of the equipment. Theoutput from this unit may then be arranged to provide the necessarycorrection such as to insert or eliminate a pulse from the incomingsignal, or modify Vthe operation of a build back circuit to produce theequivalent eiect.

For the case of a pilot frequency this unit will consist of a pulsedemodulation circuit incorporating afilter capable of selecting thecontrol frequency fc, and for the case of a diierent pulse width orduration, Va `demodulation circuit givingv a D. C. output voltage at aconstant level for correct operation, and a variationfor incorrectoperation. This D. C. output may be applied directly or by meansrof abridge circuit arrangement balanced at Vthe Voltage of correct operationto provide the requisite correcting control. f

A control unit must be incorporated at the output of each pulseseparating stage, i. e. 74, (Fig. 11),-i. e. at the output of thedifferentiating yand limiting stages, in order to ensure correct-channelselection' on starting up the receiver, or ifan interfering signal lastsfor a large number of received pulses, or alternatively if the receiversignal fades badly.

It will be appreciated that although itis only necessaryV to employ onechannel' for control purposes, more than one may beus'ed for providinggreater scope for detail design considerations.

Referring again to Fig. 10 and to Fig. l2 in which parts are given likereference to corresponding parts in Fig. 11, let it be assumed thatpulse train a has a distinguishing feature, such as a modulationfrequency fc. Now the lirst stage of selection occurs after pulse traing has been obtained and results in two separate pulse trains h and i theformer containing pulses a and c and the latter b and ld. If thecircuits for accepting a and c, i. e. 14 in Fig. 12, do so in fact, thecontrol unit 86, Fig. 12, then extracts from the a and c pulse train thecomponent of frequency fc which is applied to prevent correcting unit81, Fig. 12, from functioning. If the pulse train fed tol'fl, Fig. 12,consists of pulses b and d then no fc is obtained and unit 81, Fig. 12,applies, as explained fully hereinafter, the necessary correction to'13, Fig. 12, for example by inverting the pulse train f, Fig. 10, or byinserting a pulse in the received pulse train,

It will be seen that the pulse train accepted by 'M as correct may be a,c, a, c, etc., or c, a, c, a, etc., only one of which will provide thecorrect signals at outputs 82 and 83 etc., Fig. 12, hence the same:control operation must 4be repeated by units 88 and 89, Fig. 12, whichprovide a signal capable of causing the output from unit 'I3 to beadvanced or retarded Vtwo signal pulse periods, by the insertion orelimination of two received pulses. By causing circuit 89 to control therst build back unit 13 and not the later-'ones such as 'I6 it ispossible to ensure that'the correct signal is obtained at the outputterminals 82, 83,

` 84 and 85 respectively, by the use of only; one

pilot frequency. n

(d) By providing a distinctive modulation for each channel so that atthe receiver means may be provided for converting'this modulation intovoltage to be usedv for controlling correction circuits if a channelother than the correct one `appears at a selected channel terminalequipment. The type of correction provided depends on the cause andduration of received signal interruption. For simplicity correction willbe described for overcoming interference appertaining to thereceivedsignal and not to circuits at a later stage of the receivingequipment, Which'if deemed necessary could be a duplication of parts ofthese arrangements. This method has many advantages over the morestraight forward methods referred to under (c) and typies a type ofsynchronising method for multichannel systems employing a modulatingcomponent as opposed to a special pulse characteristic atchannel pulserepetition frequency.

A schematic diagramrof such an arrangementv is shown in Fig. 15 and inmore detail in Fig. 20. The partsin Fig. 20 'which correspond to theblocks in Fig. 15 are given the same references as the blocks. As thereare known build back circuits represented at BB available which willvoperate with positive or negative pulses, it is assumed that theappropriate ones are used, otherwise additional phase reversal deviceswill be necessary. The units comprising capacity CII` and resistance Rtogether form a -pulse differentiation circuit. DN. is a delay networkand F a filter or other arrangement for ,producing/a D'. C. controlvoltage from the characteristic of the channel, for example, the pilotfrequency. The block 96' represents the ilrst stages of a receivergivingan output similar to the train of phased pulses used for modulating thetransmitter. These are then passed through correcting unit |06 to therst build back and diil'erentiation circuit 97 after which the rstdivision takes place at units 98 and 99 and the process is repeatedgiving two further divisions at units and |0| from 93 and |02 and |03from 99. This process ymay be continued further, but by way of example,a four channel system will be described herein, as shown in Fig. 15.Units |04, |05 and |06 are the circuits provided for maintaining correctchannel selection.

For the case where a pilot frequency is used outside the required signalfrequency band, the pilot frequency wave may have a different amplitudeor frequency for each channel. In the latter case the filter forseparating out the pilot frequency may be such as to provide at theoutput thereof different amplitudes for different input frequenciesreceived. In one case, such a filter would be connected to the output ofone o channel and be provided with a rectifier circuit for producing aD. C. voltage which has a predetermined value when the correct signal isreceived and would vary when the wrong signal is received by thatchannel. Y The variation in voltage may then be used to provide thecorrection to the incoming pulse train so` that the pulses fed to thechannel having the filter provide the said predetermined datum voltagewhich exerts no control or variation in the operation of the correctingunit. 'I'his pilot frequency selection filter is indicated in Fig. 15 by|04 the output from which is fed to the two correcting units |05 and|06. Unit |05 is capable of extracting pulses from the received pulsetrain and unit |05 provides means for inserting pulses in the receivedpulse train. Such units will be described more in detail in connectionwith Fig. 19.

The necessity for providing two such types of correction units |05, |06is in order to prevent continuous over correction which is a possibilitywhen employing control voltages whose varia,

tions are slow compared to the pulse repetition frequency. Fig. 16indicates in the third row down the effect of the introduction of ashort interfering signal (INT), or in the fourth row down the loss of vapulse (the second pulse) due to fading and it will be seen that theseoccurrences are equivalent to altering the channel selection sequence.

For example, let it be assumed that unit |04 Fig. 15 is connected tochannel terminal B and that pulse train 2 provides in the outputJ ofunit |04 a voltage equal to the predetermined or datum voltage, and thenas shown in Fig. 17, the voltage produced at terminal B by the channeltrain of pulses will be less and Voltage produced by pulse trains ofchannelsV 3 and 4 greater respectively than the voltage produced by thepulse trainV of channel 2. If an interference pulse is received pulsetrain I may be received at channel B and then output Voltage of unit|04, Fig. 15, will be reduced from that produced by the correct train 2.From a study of Fig. 17 it can be seen that a pulse missed will cause anincreased voltage output while 2 pulses missed will cause a greateroutput voltage of |04, acondition which, as will be seen hereinafterwill cause a quicker correction and is especially useful where more thanfour channels are employed. 'Ihe keffect of a long period of noise, orfading may leave the system in any condition, but if circumstances aresuch as to require it there will immediately be produced in the filteroutput the necessary correcting voltage.

Units |05 and |06 may consist of a variable impedance shown as anelectron discharge tube H0, Fig. 19, whose vcontrol grid G is providedwith bias voltage from unit |04 applied at terminal |09. The receivedpulse train is applied between terminals |01 and |99 to the anode andcontrol ,grid via capacities Cl and C2 respectively.

The operation of unit |05, the pulse extractor unit, Fig. 15, can bestbe explained in conjunction with Fig. 18 and Fig. 19. (a) Fig. 18represents the input pulse train to |05, Fig. 15, at terminals |01 and|03, Fig. 19, yobtained from receiver 96, Fig. 15. vUnder normalconditions the output voltage of unit lili-Fig. 15, which is fed to |09,Fig. 19, to control 'the impedance of the tube is such as to cause theoutput of 55, Fig. 15, obtained at terminals and H2, Fig. 22, to be asshown at (b) Fig. 18.

On the occasion of an additional interference pulse being received itwill be seen that channel pulse train No. l will be received at B (seeFig. 16) so that the voltage output of |04, Fig. 15, will be reducedwhich will cause the impedance of H0, Fig. 19, to increase. The tube H5,Fig. 19, is initially biassed beyond positive cut-off, i. e. beyondsaturation point so that the resulting increased negative bias voltageon valve Vl le, Fig. 19, will cause this valve to feed an out of phasepulse to terminals H2 and lll, and the succeeding output pulses fromunit |05 will tend to decrease in amplitude. However, as the impedanceof valve H0, Fig. 19, increases, so does the time of the trailing edgesof output pulses also increase. These ,successive pulses will stilloperate unit 91, Fig; 15, until an extra large spacing between pulsesoccurs when unit Sl, Fig. 15, will not be operated, owing to the factthat the succeeding pulse occurring after the extra large spacing occursafter the preceding pulse has terminated and the amplitude of the saidsucceeding pulse is not sufficient by itself to operate the build backcircuit 01.

Due to the variable spacing which occurs between the pulses and theconsequent reduction in voltage at a relatively slow rate compared tothe mean pulse repetition frequency, pulses will only be eliminated asjust described after the longest intervals which are liable to occurbetween pulses. The chances, however, of pulse elimination will increaseuntil the requisite number of pulses have been eliminated and thecontrol voltage from |04 is increased to its normal value.

Unit |06 of Fig. 15 provides means for inserting an additional pulse orpulses into the received train of pulses and consists of a similarcircuit'to that of Fig. 19, or an impedance may be used in place ofvalve ||0 so as to produce at the terminals III, H2, a varying amplitudepulse train which is fed to an amplifier |29 (Fig. 20) that is Anormallybiassed beyond out off. Its grid bias is, however, controlled by theoutput voltage of |04, Fig. 15, so that in the event Yof this outputbecoming positiveY due to missing pulses in the received pulse train,the amplifers bias is reduced and a maximum amplitude pulse, obtainedfrom the second of two pulses occurring close together, is amplified andpassed through a delay circuit DN (Fig. 20) from which it is fed to thefirst build back circuit 91.v A nominaldelay Vshould be used in DN, andif the rst pulse coincides with another pulse, the operation will berepeated. The frequency of this operation will be increased as thecontrol voltage increases. In curve c, Fig. 18, the dotted line (i)shows the .cut off bias to which the amplifier |29, Fig. drops as theoutput from |04 increases and the continuous line shows the varyingamplitude pulses in the output of |'|l. It will be seen in thisillustrative case that only one pulse passes to amplifier |29.

y'Ihe random action of this addition and subtraction of pulses by units|06 and |05 respectively provides time for the control voltage to berestored to normal and so eliminates the possibility of excessiveover-correction. Although a low frequency control signal must inherentlytend to be relatively slow in operation, by correct adjustment of thecircuit time constants involved, it is possible to obtain very rapidaction with the arrangement described as use is made of the iirstinitialchange only. fi .i ,1

The advantages of employing a distinctive feature of the channelmodulations as compared with the employment of additional pulses forsynchronisation are a reduction of the overall band width oftransmission with a saving in radiated energy, and also it would mostprobably permit of `an increase inthe maximum permissible modulation perchannel.

A further point applicable to the system herein described -is that it ispossible to adjust unit |05, Figs. 15 and 20, so that pulses of ashorter duration than thecorrect pulses will not be passed through itwith su'icient amplitude to operate thefirst build back circuit.

All that is necessary to convert a phased pulse system as hereinbeioredescribed, into a double pulse system is the provision at the receivingend of the fixed marking pulses which may readily be accomplished by anyof the following methods.

Firstly, to the master oscillator ci the channel pulse frequency,obtained by any of the previously described methods may be added furtherphasing circuits'for generating the fixed marking pulses.

Secondly,v in the system requiring a pulse-for controlling vpasscircuits, this pulse should be increased in duration and phased so as topermit both the received phased pulses and the inserted Xed pulses to bepassed. The fixed pulses are obtained by multiplication from the masteroscillator i. e. if four channels are required, then the fourth harmonicis generated and the correspondingl pulses obtained therefrom appliedsimultaneously to each channel circuit, one pulse of every four beingaccepted by each channel in turn.

Whilst an antenna has been shown, the pulses may, it will be understood,be received over a transmission line.

Another method, employing the progressive selection principle has onechannel unmodulated and then the pulses in this Vchannel are eitherphased by time delay networks or by extracting a Wave of channel pulserepetition frequency and obtaining from it pulses which are properlyphased by phasing circuits for the respective channels.

This principle could, of course, be employed with any system of phasedpulses.

In the case where a large number of channels are required, and afrequency multiplication stage is employed, it may be found an advantageto sub-divide the channels into groups.

It will be understood that the methods and arrangements describedhereinbefore for directing the pulses to their respective channels atthe receiver may also equally well be employed in a` multiplex systemutilising duration modulated pulses in place of the time modulated orphased pulses, with the exception of the progressive selection systemwhich involves differentiation circuits. The signals may be obtained ineach channel from the duration modulated pulses in known manner.

` What isclaimed is:

1. In a multichannel electrical signallingl sys-V tem utilizingthe-transmission of electrical pulses, each channel comprising a trainof time-phase modulated pulses, time-phased with respect to the pulsesof other channels, means for directing the pulse trains of the-channelsto respective receiving apparatus including progressive selection meansfor deriving from the received total pulse train of all .the channels aparticular channel pulse train comprising a iirst stage including buildback circuit means for deriving from consecutive pairs of receivedpulses further pulses of durations equal to the time intervals betweenthe received pulses of the respective pairs, means for differentiatingsaid further pulses, means for eliminating pulses otone sign from thepulses resulting from the differentiation means, and further stagesincluding bui-ld back circuit means for obtaining in like manner fromconsecutive pairs of the selected diierentiated pulse train vof thepreceding stage further pulse trains which comprise pulses of a singlechannel only, and means in each receiving apparatus for deriving as anamplitude modulated wave from the time-phase modulation of the train ofpulses` receive the intelligence carried thereby. v

2. In a multichannel electrical signalling system as claimed in claim 1,progressive selection means wherein said means for deriving from thereceived total pulse train or from a train of resulting differentiatedpulses comprises a build I back circuit arrangement for producing pulseswhose durations'are equal to the time intervals between the `pulses ofthe respective pairs of pulses. f f

3. Progressive selection means asr claimed in claim l furthercomprisingcorrecting means for ensuring correct channel selection of pulses fromthe received pulse train. Y l f 4. Progressive selection means asclaimed in claim 1` further rcomprising `correcting means ins corporatedat the output of each pulse separating stagefor ensuring correct channelselection of pulses from the received pulse train.

5. Progressive selection means as claimed in claim 1 further comprisingcorrecting means incorporated at the output of each pulse separatingstage for ensuring correct channel selection of pulses from the receivedypulse train, said means being designed to insert a pulse or toeliminate a pulse from the received pulse train at the Y input to apulse build back circuit means.

6. In a multi-channel electrical signalling system utilizing thetransmission of electrical pulses, each channel comprising a train oftime-phase modulated pulses, time-phased with respect to the pulses ofother channels, receiving apparatus comprising means for directing thepulse trains of the channels to respective channel receiving circuitsincluding'progressive selection means f-or deriving from the receivedtotal pulse train of all the channels a particular channel pulse traincomprising a iirst stage including build back circuit means for derivingfrom consecutive pairs of` received pulses further pulses of durationsequal to the time intervals between the received pulses of therespective pairs, means for differentiating said further pulses, meansfor eliminating pulses of one sign from the pulses resulting from thedifferentiation means, further stages including build back circuit`means for obtaining in like manner from consecutive pairs of theselected differentiated pulse train of the preceding stage further pulsetrains which comprise pulses of a single channel only, and' correctingmeans incorporated at the output of each pulse separating stage forensuring correct channel selection of pulses from the received pulsetrain, said correcting means comprising a control unit and a correctingunit, said control unit being responsive to a characteristic of thereceived pulses and said correcting unit comprising means for insertinga pulse inthe received pulsetrain and means for eliminating a pulse inthe received pulse train under the control of said control unit, andmeans in each channel receiving circuit for deriving as an'amplitudemodulated Wave from the timephase modulation of the train of pulsesreceived the intelligence carried thereby.

7. Progressive selection means as claimed in claim 6 wherein saidcontrol unit comprises means for giving a variable output voltagedepending upon the variability of predetermined characteristic of thepulses in a channel.

8. Progressive selection means as claimed in claim 6 wherein said meansfor eliminating a pulse from the received pulse train comprises avariable impedance, means for varying said impedance under the controlof said control unit, means for applying the received pulses to theinput terminals of said impedance, and means for applying the outputpulses to the build back circuit means.

9. ProgressiveV selection means as claimed in claim 6 wherein said meansfor inserting a pulse into the received pulse train comprises animpedance, means for applying the received pulses to the input terminalsof said impedance, an amplifier, 'means for varying the gain of said amlplier under the control of said control unit, means for feeding theoutput from said imped ance to said amplifier, and means for feeding theoutput of said ampliiier through a delay network of appropriate delayconstant to the build back circuit means.

v10'. Progressive selection means as claimed in claim 6 wherein themeans for eliminating a pulse from the received pulse train comprises avar- 16 iable impedance electron discharge tube, and wherein is providedmeans for varying the impedance oi said tube under the control of saidcontrol unit, means for applying the received pulses of the inputterminals of said tube, and means for applying the pulses from theoutput of said tube to the build back circuit means.

' PRAFULLA KUMAR CHATTERJEA.

LESLIE WILFRED HOUGHTON.

REFERENCES CITED Number Name Date 1,928,093 Coyle Sept. 26, 19332,021,743 Nicolson Nov. 19, 1935 2,036,350 Montani Apr. 7, 19362,048,081 Riggs July 21, 1936 2,057,773 Finch Oct. 20, 1936 2,061,734Kell Nov. 24, 1936 2,085,418 Crosby June 29, 1937 2,100,156 BushbeckNov. 23, 1937 2,110,548 Finch Mar. 8, 1938 2,113,214 Luck Apr. 5, 19382,146,876 Zworykin Feb. 14, 1939 2,171,150 Shelby Aug. 29, 19392,172,354 Blumlein Sept. 12, 1939 2,185,693 Mertz Jan. 2, 1940 2,199,634Koch May 7, 1940 2,213,941 Peterson Sept. 3, 1940 2,256,336 Beatty Sept.16, 1941 2,259,392 Roberts Oct. 14, 1941 $2,262,838 Deloraine et al Nov.18, 1941 2,265,216 Wolf Dec. 9, 1941 2,266,401 Reeves Dec. 16, 19412,275,974 Mathes Mar. 10, 1942 2,277,192 Wilson Mar. 24, 1942 2,282,046Goldsmith May 5, 1942 2,284,401 Manley et al May 26, 1942 2,308,639Beatty et al Jan. 19, 1943 2,328,944 Beatty Sept. 7, 1943 2,403,210Butement et al July 2, 1946 FOREIGN PATENTS Number Country Date 362,943Great Britain Dec. 11, 1931

